ZnapZend 0.21.0

• updated install instructions see README.md

• new options for znapzend

  --forbidDestRollback forbids forcing destination dataset rollback and
  removal of datasets/snapshots not present on source

  --since=x allow to consider a non-automatic common snapshot "x" as a
  starting point

  --sinceForced=x same as --since=x but also make sure "x" exists on
  destination (can cause removal of newer snapshots)

  --cleanOffline clean up source snapshots even if a destination was offline

  --mailErrorSummaryTo=rcpt if "send task(s) failed", mail a summary to rcpt

• new --feature=x flags for znapzend sendIntermediates forbidDestRollback

• an a ton of bugfixes for all sorts of edge cases

A big Thank You! to Jim Klimov @jimkilmov who contributed a lot of time
and code to this release